This invention relates to a process for the activation of dispersed active metal catalysts that enhances their activity and selectivity in the production of higher hydrocarbons from synthesis gas.
The production of higher hydrocarbon materials from synthesis gas, i.e. carbon monoxide and hydrogen, commonly known as the Fischer-Tropsch (xe2x80x9cF-Txe2x80x9d) process, has been in commercial use for many years. Such processes rely on specialized catalysts. The original catalysts for the Fischer-Tropsch synthesis were nickel. Nickel is still the preferred catalyst for hydrogenation of fats and specialty chemicals. Over the years, other metals, particularly iron and cobalt, have been preferred in the Fischer-Tropsch synthesis of higher hydrocarbons whereas copper has been the catalyst of choice for alcohol synthesis. Cobalt is particularly preferred for Fischer-Tropsch synthesis due its high productivity and comparatively low methane selectivity. As the technology of these syntheses developed over the years, the catalysts became more refined and were augmented by other metals and/or metal oxides that function to promote their catalytic activity. These promoter metals include the Group VIII metals, such as platinum, palladium, rhenium, ruthenium and iridium. Metal oxide promoters include the oxides of a broader range of metals, such as molybdenum, tungsten, zirconium, magnesium, manganese and titanium. Those of ordinary skill in the art will appreciate that the choice of a particular metal or alloy for fabricating a catalyst to be utilized in Fischer-Tropsch synthesis will depend in large measure on the desired product or products.
Particularly suited for the production of hydrocarbons by Fischer-Tropsch synthesis from synthesis gas are Dispersed Active Metals (xe2x80x9cDAMxe2x80x9d) which are primarily, i.e. at least about 50 wt. %, preferably at least 80 Wt. %, composed of one or a mixture of metals such as described above and are, without further treatment, capable of catalyzing Fischer-Tropsch synthesis. DAM catalysts may be prepared by any of a number of art-recognized processes.
In 1924, M. Raney prepared a nickel hydrogenation catalyst by a process known today as the Raney Process. For purposes of simplicity, the term xe2x80x9cRaneyxe2x80x9d will be utilized herein as a generic term to describe the process, alloys and catalysts obtained thereby. This specific synthesis, in essence, comprises forming at least a binary alloy of metals, at least one of which can be extracted, and extracting it thereby leaving a porous residue of the non-soluble metal or metals that possesses catalytic activity. The residue, or non-extractable, catalyst metals are well known to those skilled in the art and include Ni, Co, Cu, Fe and the Group VIII noble metals. Likewise, the leachable or soluble metal group is well known and includes aluminum, zinc, titanium and silicon, typically aluminum. Once alloys are formed of at least one member of each of these groups of metals, they are ground to a fine powder and treated with strong caustic, such as sodium hydroxide, to leach the soluble metal.
There exist many variations of the basic preparation of Raney catalysts such as, for example, deposition of alloys onto a performed support by flame spraying, (U.S. Pat. No. 4,089,812), formation of the alloy by surface diffusion of aluminum on a non-leachable metal substrate (U.S. Pat. No. 2,583,619), and forming pellets from the powdered alloys for use in fixed bed reactions vessels (U.S. Pat. Nos. 4,826,799, 4,895,994 and 5,536,694). These developments have made possible the use of shaped Raney catalysts in fixed bed reaction vessels.
A preferred reactor carrying out for Fischer-Tropsch reactions utilizing DAM catalysts is the slurry bubble column developed by Exxon Research and Engineering Company. This reactor, which is ideally suited for carrying out highly exothermic, three-phase catalytic reactions, is described in U.S. Pat. No. 5,348,982. In such reactors, the solid phase catalyst is dispersed or held in suspension in a liquid phase by a gas phase that continuously bubbles through the liquid phase. The catalyst loading in slurry bubble reactors can vary within a broad range of concentrations, but must remain short of the so-termed xe2x80x9cmud limitxe2x80x9d where the concentration becomes so high that mixing and pumping of the slurry become so difficult that practical operation is no longer possible. The use of high metal-loading catalysts or bulk metal catalysts is preferred in slurry bubble reactors in order to maximize the productivity of both catalyst and reactor.
An extensive review of process of forming DAM catalysts can be found in xe2x80x9cActive Metalsxe2x80x9d, Edited by Alois Furstner, published by VCH Verlagsgesellschaft mbH, D-69451 Weinheim (FRG) in 1996 and the references cited therein. Methodologies described therein include the Reike method, the use of ultrasound, reduction of metal salts, colloids, nanoscale cluster and powders. Other relevant references include, for example, the preparation of amorphous iron catalyst by high intensity sonolysis of iron pentacarbonyl, Suslick et al., Nature, Vol. 353, pp 414-416 (1991) and the formation of single domain cobalt clusters by reduction of a cobalt salt with hydrazine, Gibson et el., Science, Vol. 267, pp 1338-1340, (1998). Finally, intermetallic alloys, particularly those known for forming metal hydrides, such as LaCo5, can be formed into a fine powder by the application of hydrogen adsorption/desorption cycles. DAM catalysts can also be prepared by thermal or chemical decomposition of metal formates or oxalates. These methods are given as examples and are not intended in any way to limit the term xe2x80x9cDAMxe2x80x9d as utilized in the context of the present invention.
One of the primary characteristics of DAM catalysts is that, in their dry form, they are generally pyrophoric. For this reason, they are generally stored and shipped in airtight containers, typically as a slurry in an appropriate solvent, such as water or oil, or coated with a removable protective layer of an air-impervious material, such as wax. We are not aware of any DAM catalysts that are not used as they are formed, i.e. without no further treatment following extraction of the leachable metal and subsequent drying steps as described above. On the opposite end of the cycle, the manufacturers of DAMs recommend that spent catalysts, i.e. those no longer economically effective, must undergo deactivation in order that they may be safely disposed of. Such deactivation is generally achieved via oxidation of the metal by air oxidation or treatment with dilute bleach solution.
It will be appreciated that a means of enhancing the activity of the catalyst would greatly increase its value in the process. Another important aspect of the value of a catalyst is its selectivity which is the ratio of the percent of feed material converted to desired higher hydrocarbons to that of short chain hydrocarbons produced, primarily methane, commonly referred to as xe2x80x9cmethane selectivityxe2x80x9d. In accordance with the present invention, there is provided a process that not only significantly enhances the activity of DAM catalysts, but markedly enhances their methane selectivity as well.
In accordance with the present invention, Dispersed Active Metal (xe2x80x9cDAMxe2x80x9d) Fischer-Tropsch catalysts are enhanced both in activity and methane selectivity by low temperature oxidative deactivation in a slurry phase to form an oxidized catalyst precursor comprising said metals and at least one of hydroxides thereof and oxides thereof followed by reductive reactivation with hydrogen at elevated temperature. The present invention also includes the use of slurry low temperature oxidation to form an oxidized catalyst precursor that differs compositionally from that obtained by conventional high temperature oxidation utilizing an oxygen-containing gas.
It is well known to those skilled in the art of Fischer-Tropsch synthesis chemistry that Group VIII metal surfaces exhibit higher activities for catalytic reactions such as hydrogenation, methanation and Fischer-Tropsch synthesis when subjected to a high temperature oxidation-reduction (O-R) cycle. Such xe2x80x9cactivationxe2x80x9d techniques are reviewed in Applied Catalysis, A. General 175, pp 113-120 (1998) and citations therein. A series of patents, e.g. U.S. Pat. Nos. 4,492,774; 4,399,234; 4,585,789 and 4,670,414 disclose activation of a cobalt catalyst by a reduction/oxidation/reduction (R-O-R) cycle. So far as we are aware, all such oxidation/reduction and reduction/oxidation/reduction cycles described in the literature are effected by treating a solid catalyst with an oxygen-containing gas at high temperatures. This treatment results in the formation of the most stable oxide of the metal, i.e. in the instance of cobalt, Co3O4. All DAMs treated in accordance with the invention are characterized by the capacity to exist in more than one oxidation state and, thereby, form more than one oxide. Heretofore, the process described above have sought to completely oxidize such DAMs to the highest oxidation state oxide which corresponds to the most stable oxide.
In the activation treatments described above, the oxygen content of the treating gas in the oxidation step varies from as low as 15 ppm to pure oxygen and the temperatures typically are between about 200xc2x0 C. to 600xc2x0 C. Several publications dealing with these activation methodologies also stress the importance of controlling the exothermicity of the reaction to avoid sintering of the cobalt oxide particles since that may be detrimental to the activity of the final catalyst. We have found that this latter observation is even more critical with regard to the oxidation of DAM catalysts because of their high metal content, particularly those that may also contain active hydrogen species as in Raney catalysts or metal hydrides.
It has been found in accordance with the present invention that significant enhancement in both the activity and methane selectivity for Fischer-Tropsch synthesis is realized by treating a DAM catalyst with an oxidation/reduction cycle wherein the oxidation is carried out in a slurry phase at low temperature. By low temperature is meant a temperature below 200xc2x0 C., preferably below 100xc2x0 C. The oxidation is effected by bubbling a gaseous oxidant through a slurry of the DAM catalyst, or by the slurry itself formed from or combined with an aqueous solution of a suitable oxidant. Typical conditions for the oxidative deactivation of a DAM catalyst in accordance with the present invention utilizing an oxidative gas are as follows: ratio of liquid to DAM by volumexe2x80x94at least about 3:1, preferably at least about 5:1: temperaturexe2x80x94from about 25xc2x0 C. to 100xc2x0 C., preferably from about 50xc2x0 C. to 80xc2x0 C.; total pressurexe2x80x94from about 15 to 300 psia, preferably from about 15 to 100 psia; contact time for the DAM in the slurryxe2x80x94at least one hour, preferably until the DAM has lost pyrophoricity; and gas flow ratexe2x80x94at least 100 cc/min. Typical oxidative gases in addition to oxygen include ozone and nitrogen oxides, i.e. nitrous oxide and nitric oxide, all of which may be utilized in pure form, but typically are mixed with one or more inert diluent gases. Wherein oxygen is utilized, for example, typically air is caused to flow into the slurry. Alternatively, pure oxygen can be mixed with an inert gas in from about 1 to 50%, preferably from about 5 to 25% by volume.
Wherein the oxidative treatment in accordance with the present invention is carried out utilizing a dilute solution of an oxidant, the oxidant is chosen so as not to introduce substances into the slurry that are recognized as being permanent poisons of the Fischer-Tropsch synthesis, e.g. ionic forms of chlorine, bromine, phosphorus and sulfur. Included within the scope of oxidants in solution are solutions of compounds that form oxidants in situ upon contact with air, for example, certain alcohols will form hydroperoxides upon contact with air. Preferred oxidants include nitric acid and inorganic nitrates, for example, ammonium nitrate, hydrogen peroxide, and art-recognized organic peroxides or hydroperoxides. Those skilled in the art will appreciate that the concentration of individual oxidants will vary according to their oxidizing capacity. In general, the amount of the oxidant in the slurry and the duration of the oxidation are sufficient to insure oxidation to a point such that the resulting dry DAM material would not exhibit uncontrollable pyrophoric responses upon exposure to ambient air and moisture but not so great as to cause unwanted secondary reactions such as dissolution or extraction of the active metal ions in the catalyst.
The liquid utilized to form the slurry in accordance with the present process is preferably water. It is within the scope of the process, however, to utilize organic solvents that do not introduce any known poison of the Fischer-Tropsch synthesis and that are non-reactive with the conditions of the oxidation treatment. Hydrocarbons, particularly those derived from the Fischer-Tropsch synthesis itself are appropriate and may be used with either an oxygen-containing gas or dilute solution of the oxidants named above that are soluble therein, such as the organic peroxides. Further, mixtures of water and organic solvents miscible therewith can be utilized as well. Mixtures of water with immiscible solvents can also be utilized in combination with suitable dispersing or emulsifying agents present to form a continuous phase, i.e. an emulsion. Other suitable liquids include dense fluids, for example, supercritical fluids such as liquid phase light, i.e. C3-5 alkanes, cyclopentane and the like. Preferred mixed liquids include, without any intended limitation, water/lower alkanols, water/Fischer-Tropsch products, and water/alkanols/alkanes.
The oxidative treatment in accordance with the present invention may be carried out in any reactor apparatus suitable for slurry reactions including, with no limitation intended, fixed bed reactors, moving bed reactors, fluidized bed reactors, slurry reactors, bubbling bed reactors and the like. Irrespective of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the slurry will typically be somewhere between the theoretical limiting conditions of plug flow and complete back mixing.
The product of the low temperature oxidation treatment of a DAM in accordance with the present invention is a mixture of metallic and oxidic species. This is the result of the fact that the metals in the DAMs can exist in more than one oxidation state and, in the treatment of the invention, a significant portion of the active metal of the DAM is oxidized to a lower oxidation state. In contrast, the prior art high temperature oxidation treatments result in complete oxidation of the active metal to the highest, and most stable, oxidation state. For example, in the treatment of the present invention, a significant portion of cobalt metal is oxidized to CoO and/or Co(OH)2 rather than Co3O4, iron metal is oxidized to FeO and/or Fe(OH)2 rather than Fe3O4. Additionally, when the slurry in which the treatment of the invention is effected contains water, hydroxides of the metals will be formed as part of the mixture referred to above. This mixture is in fact an oxidized catalyst precursor wherein, on a mole percent basis, not more than 50% of the active metal present is in the form of the oxide of the highest oxidation state, and the highest oxidation state of the metal in combination with the amount in the metallic state does not exceed 85% of the active metal present, the remainder being lower oxidation state oxides and/or hydroxides. Preferably, not more than 25% of the active metal present is in the form of the oxide of the highest oxidation state, and the highest oxidation state of the metal in combination with the amount in the metallic state does not exceed 60% of the active metal present, the remainder being lower oxidation state oxides and/or hydroxides.
Upon completion of the oxidation treatment in accordance with the process of the invention, i.e. when the DAM no longer exhibits uncontrollable pyrophoricity, the resulting oxidized catalyst precursor is recovered from the slurry and dried. By not exhibiting uncontrollable pyrophoricity is meant that, upon filtering the DAM in air, the temperature should not rise above 200xc2x0 C. The precursor is then converted to the active catalyst by reduction with hydrogen-containing gas at temperatures of from about 200xc2x0 C. to 600xc2x0 C., preferably from about 300xc2x0 C. to 450xc2x0 C., most preferably from about 340xc2x0 C. to 400xc2x0 C. Hydrogen partial pressure during the reduction would range from about 1 to 100 atmospheres, preferably from about 1 to 40 atmospheres. Typical Fischer-Tropsch activities of DAM catalysts activated in accordance with the process of the present invention are at least 120%, more frequently at least 150%, of that of the original DAM. By the same token, methane selectivity of the DAMs are reduced by the present process to below 80%, more frequently below 60% of the original DAM. As those of ordinary skill in the art are aware, the most desirable products of the Fischer-Tropsch synthesis described herein are higher molecular weight hydrocarbons. By contrast, therefore, it is desirable for there to be produced as little methane as possible. Hence, methane selectivity is enhanced when the percentage is reduced, therefore, a reduction in methane selectivity is a significant improvement.
The process of the present invention is obviously commercially attractive since it significantly enhances both the activity and the methane selectivity of the active metal catalyst. It is also within the purview of the present invention to carry out the two steps of the activation at different times since the pyrophoricity of the catalyst has been significantly reduced after the slurry low temperature oxidation to form the oxidized catalyst precursor in accordance with the present invention. The oxidized catalyst precursor may be shipped in conventional airtight containers under an inert atmosphere, such as nitrogen. Preferably, the catalyst oxidized in the slurry is further stabilized by treatment in air at a temperature above 100xc2x0 C. for at least one hour, which may eliminate the requirement for shipping under an inert atmosphere. In either instance, there is a saving of the substantial cost presently incurred in shipping conventional active metal catalysts under water or occlusive coatings. The oxidative treatment of the present invention is also useful in enhancing the activity of DAM catalyst that has been withdrawn from an operating Fischer-Tropsch reactor. The present process may likewise be used to further enhance DAM catalyst that has been withdrawn from an operating Fischer-Tropsch reactor and renewed through the formation of a melt.
The catalysts formed from DAMs in accordance with the activation process of the invention are used in synthesis processes for the formation of higher hydrocarbons wherein liquid and gaseous products are formed by contacting a syngas comprising a mixture of hydrogen and carbon monoxide with shifting or non-shifting conditions, preferably the latter in which little or no water gas shift takes place. The process is carried out at temperatures of from about 160xc2x0 C. to 260xc2x0 C., pressures of from about 5 atm to about 100 atm, preferably from 10 to 40 atm, and gas space velocities of from about 300 V/Hr/V to about 20,000 V/Hr/V, preferably from about 1,000 V/Hr/V to about 15,000 V/Hr/V. The stoichiometric ratio of hydrogen to carbon monoxide is about 2.1:1 for the production of higher hydrocarbons. This ratio can vary from about 1:1 to 4:1, preferably from 1.5:1 to 2.5:1, more preferably from 1.8:1 to 2.2:1. These reaction conditions are well known to those skilled in the art and a particular set of reaction conditions can readily be determined from the parameters given herein. The reaction may be carried out in virtually any type of reactor, e.g. fixed bed, moving bed, fluidized bed and the like. The hydrocarbon-containing products formed in the process are essentially sulfur and nitrogen free.
The hydrocarbons produced in a process as described above are typically upgraded to more valuable products by subjecting all or a portion of the C5+hydrocarbons to fractionation and/or conversion. By xe2x80x9cconversionxe2x80x9d is meant one or more operations in which the molecular structure of at least a portion of the hydrocarbon is changed and includes both non-catalytic processing, e.g. steam cracking, and catalytic processing, e.g. catalytic cracking, in which the portion, or fraction, is contacted with a suitable catalyst. If hydrogen is present as a reactant, such process steps are typically referred to as hydroconversion and variously as hydroisomerization, hydrocracking, hydrodewaxing, hydrorefining and the like. More rigorous hydrorefining is typically referred to as hydrotreating. These reactions are conducted under conditions well documented in the literature for the hydroconversion of hydrocarbon feeds, including hydrocarbon feeds rich in paraffins. Illustrative, but non-limiting, examples of more valuable products from such feeds by these processes include synthetic crude oil, liquid fuel, emulsions, purified olefins, solvents, monomers or polymers, lubricant oils, medicinal oils, waxy hydrocarbons, various nitrogen- or oxygen-containing products and the like. Examples of liquid fuels includes gasoline, diesel fuel and jet fuel, while lubricating oil includes automotive oil, jet oil, turbine oil and the like. Industrial oils include well drilling fluids, agricultural oils, heat transfer oils and the like.